They are custom-design Cinemag, optimized for our circuit. They are not off-the-shelf parts, which have bandwidth problems with the output impedance of the 6SN7.
The 6SN7 is one of the best tubes ever made, but is not easy to match with most transformers. That’s where Cinemag came in, who saw the 6SN7 as a fun challenge. Several prototypes later (design, computer model, build, measure, listen, and repeat the cycle), we arrived at the production models we’re using now.
The Khozmo volume control is also optimized for this preamp, with a different signal path than most preamps. The Raven is balanced throughout, from input to source selector to volume control to vacuum tube to output.
One nice thing about transformer coupled balanced construction is the risk of sending DC pulses to a delicate transistor power amp are greatly reduced. (Even when a transistor amp is switched off, a DC pulse of more than a few volts can damage the input transistors.)
P.S. How does the output transformer protect a power amp? First, there’s a 4.5 times step-down ratio, reducing unwanted transients by a similar ratio. Second, the circuit itself is balanced, instead of a single-ended cathode follower exposed to a hundred volts or more. Circuit balance is typically 3% or better, reducing potential transients by a similar amount (about 30 dB). Third, and most important, transformers can never pass DC, unless the windings themselves have failed. By contrast, capacitors may pass "leakage current" and gradually short out as they age (a well-known problem when restoring vintage electronics).